In chapter Exploration of the game world, I have mentioned, among other things, that the successive locations in Dragon Age: Inquisition become available gradually as you progress in the storyline. A good idea is to plan on your progress, thanks to which you will not progress too fast to the next location, where there are more demanding opponents to defeat and, on the other hand, you will not miss out on optional locations, where you can obtain more loot, experience or, e.g. new allies.
At the beginning of the game (after you complete the Prologue), try to explore the entire Haven thoroughly (the village that is the first HQ for the Inquisition). What speaks for this, is that you can find the first crafting materials there, complete quests or collect notes, thanks to which you will progress by 1-2 experience levels in the very beginning.
The first big location that you gain access to is the Hinterlands. Just like in the case of the Haven, it is a good thing to explore the location thoroughly and pay attention to loot chests and the exclamation marks that mark the starting points for side quests (there are lots and lots of them in the Hinterlands).
Unfortunately, not all of the Hinterlands will be available from the very beginning. Some of the areas in this location (e.g. the Redcliffe village) remain unavailable until you complete some of the main quests. An even greater handicap is the facts that the rims of the Hinterlands are being occupied by much stronger monsters (especially ones at level 8 or 12), which your party may not be able to go against. Therefore, it is better to leave the exploration and ultimate "securing" of the area for later.
If possible, I recommend that you complete several initial main quests quite early, at least up to the moment at which you decide whether to join the Templars or the rebellious mages. In the meantime, you will gain access to Val Royeaux and several bigger optional locations. I especially recommend that you journey to The Storm Coast, where you will be able to include the Iron Bull in your party. Apart from that, you can also visit The Forbidden Oasis.
I recommend that, later into the game, you explore the newly unlocked areas and complete new main quests. Each time, before you start a "big" main quest, make sure to check the experience level recommended for the quest. If, for example, it is not recommended to start a given quest below experience level 10, spend more time in the additional areas of the world to gain some experience. This will prevent (SERIOUS) problems with completing such a quest.
NOTE! You can also find the recommended experience levels for the individual missions in Dragon Age: Inquisition in the walkthrough for the game.